A. Field of the Invention
This invention relates to a liquid crystal display device and, more particularly, a reflective-type liquid crystal display device.
B. Description of the Prior Art
Liquid crystal display devices (“LCDs”) can be classified into TN (Twisted Nematic) type, GH (Guest Host) type, ECB (Electrically Controlled Birefringence) type and OCB (Optically Compensated Birefringence) type and so on, in accordance with the driving mode.
Recently, the TN-LCDs have been extensively used in devices such as the portable personal computer, measurement apparatus and the like. However, TN-LCDs, while having a number of advantages, also have the disadvantages of a narrow viewing angle and low response time.
When a voltage is applied to a liquid crystal cell, the birefringence index of the liquid crystal cell changes as the alignment of the liquid crystal molecules changes corresponding to the dielectric anisotropy of the liquid crystal.
A mode, called the ECB mode, induces a change in the light transmission to change the birefringence index of the liquid crystal cell. HAN (Hybrid-Aligned Nematic) mode LCDs are representative of the ECB-LCDs. HAN mode LCD's have the advantages of having a low driving voltage and the ability to display full color well. Studies of the HAN mode have been widely advanced in OCB mode LCDs using the optical change of the liquid crystal cell as well as ECB mode LCDs.
FIG. 1 is drawing showing a schematic structure of a traditional reflective-type OCB-LCD (hereinafter ROCB-LCD) using HAN mode. The ROCB-LCD includes a first substrate 11 having a reflective electrode 16, a second substrate 12 onto which a biaxial retardation film 14 and a polarizer 15 are formed, and a liquid crystal layer 10 between first substrate 11 and second substrate 12. Movement of the liquid crystal display device having above-discussed structure will be explained in detail.
After incident light (not shown) passes through polarizer 15, biaxial retardation film 14, second substrate 12, and the liquid crystal layer 10 to compensate for phase changes due to viewing angles of a user, the incident light is reflected by reflective electrode 16 formed on first substrate 11. It is thereby possible for the user to achieve a desired image.
The above traditional HAN mode ROCB-LCDs, unlike the TN-LCD, have no restrictive condition to satisfy the first minimum condition regarding the viewing angle problems, i.e., Δn·d≧λ/2 (where Δn is the refractive index, d is a thickness of the liquid crystal layer, and λ is the wavelength of light), and since it is a SB(semi-bend) structure, there is no phase change of splay/bend. In the above HAN mode ROCB-LCD, a front scattering film can be attached on the polarizer 15, or a plurality of convex portions can be formed on the refractive electrode 16 in order to achieve a wide viewing angle.
The liquid crystal used in traditional HAN mode ROCB-LCDs has different light paths according to the y-z plane (including director) and x-z plane. In order to compensate the phase difference corresponding to the change in viewing angle of the user, the difference in the light path must be compensated by using a biaxial retardation film which has three reflective indices at the x, y, and z axes.
Therefore, the HAN mode ROCB-LCD using the biaxial retardation film has many advantages over the TN-LCD. However, gray is introduced by the decreased retardation of the liquid crystal when a voltage is applied to it. Further, the use of biaxial retardation films is disadvantageous due to manufacturing cost and the need to satisfy the complex condition demanded by the user.